The production of circuit boards typically includes loading the circuit boards onto a conveyor line and transferring them down the conveyor line to a station where the leads of electrical components are inserted into the holes through the circuit board by a machine and/or manually. The boards are then transferred to an edge handling conveyor which transfers the boards through a flux application station where solder flux is applied to the circuit boards so that the leads of the electrical components can later be soldered to the metallized areas of the board with a high quality solder bond. After moving through the flux application station, the boards are transferred through a preheat zone to flash off the solvents from the flux and to preheat the board to minimize thermal shock from contact with solder wave. Continuing the process, the board is transported through a wave solder machine where the board moves over a wave of solder. The solder is drawn or forced up into through-holes containing the leads of the electrical components and solder bonds between the leads and the metallized sections of the board are formed. After leaving the wave solder machine, the board is sent through a cleaning machine to remove the residue left from the flux, if necessary. The effectiveness of the flux application, the type of flux being applied, the need to clean the board subsequent to soldering, and the need to clean the coating chamber in which the solder flux is applied, each present problems which are addressed by the present invention.
As described in U.S. Pat. No. 5,415,337, assigned to Nordson Corporation of Westlake, Ohio, the assignee of the present invention, which is incorporated in its entirety herein, low-solid fluxes, or "no-clean fluxes", which contain small amounts, e.g., about 1% to 5% by weight of solids (activator and vehicle) and the remainder liquid solvent, such is isopropyl alcohol, are being increasingly used by circuit board manufacturers in an effort to eliminate the need to clean the circuit boards after soldering. Because of the small amount of solids within no-clean fluxes, the amount of residue left on the board is significantly reduced, as compared to the amount of residue remaining after the use of conventional rosin-based fluxes. These low-solid, no clean fluxes are particularly attractive because, as their name implies, and flux cleaning of the circuit boards after soldering is not required, which results in a significant cost savings.
The application of the no-clean fluxes using the control system described in U.S. Pat. No. 5,415,337 has sometimes led to problems because the controller is difficult to program for boards of varying length, for boards requiring more than one spray profile, for varying speeds of the conveyor, for multiple boards carried on pallets, and for spraying selected sections of the boards.
The problem with the control system described in U.S. Pat. No. 5,415,337 is that the control system does not always initiate the spray cycle for boards or sometimes initiates the spray cycle when a board is not in place. The latter problem generates overspray within the coating chamber. This overspray wastes flux coating material and results in the chamber and the conveyor becoming covered with the flux coating material and being difficult and time consuming to clean. Also, the overspray tends to clog up the control sensor which signals the location of the circuit board to the control system. This is detrimental because the spray is then turned on when the board is not in the proper location. This exacerbates the problem by generating even more overspray and creating an even greater mess which requires more frequent cleaning. Whenever the system is cleaned, the entire line must be shutdown, further increasing the overall manufacturing costs of the circuit boards.
Therefore, there is a need for an easily programmed controller able to control a spray gun for applying solder fluxes, and particularly low-solid fluxes, to circuit boards so that an even coating of the solder flux is applied to the entire board irrespective of the speed of the conveyor line or the length of the board. In addition, there is a need for a controller able to control a gun for applying coatings of varying length and thickness to different zones of a singled board.